NMR Chemical Shifts of Common Laboratory Solvents as Trace Impurities

Transcription

1 7512 J. Org. Chem. 1997, 62, NMR Chemical Shifts of Common Laboratory Solvents as Trace Impurities Hugo E. Gottlieb,* Vadim Kotlyar, and Abraham Nudelman* Department of Chemistry, Bar-Ilan University, Ramat-Gan 52900, Israel Received June 27, 1997 In the course of the routine use of NMR as an aid for organic chemistry, a day-to-day problem is the identification of signals deriving from common contaminants (water, solvents, stabilizers, oils) in less-than-analytically-pure samples. This data may be available in the literature, but the time involved in searching for it may be considerable. Another issue is the concentration dependence of chemical shifts (especially 1 H); results obtained two or three decades ago usually refer to much more concentrated samples, and run at lower magnetic fields, than today s practice. We therefore decided to collect 1 H and 13 C chemical shifts of what are, in our experience, the most popular extra peaks in a variety of commonly used NMR solvents, in the hope that this will be of assistance to the practicing chemist. Experimental Section NMR spectra were taken in a Bruker DPX-300 instrument (300.1 and 75.5 MHz for 1 H and 13 C, respectively). Unless otherwise indicated, all were run at room temperature (24 ( 1 C). For the experiments in the last section of this paper, probe temperatures were measured with a calibrated Eurotherm 840/T digital thermometer, connected to a thermocouple which was introduced into an NMR tube filled with mineral oil to approximately the same level as a typical sample. At each temperature, the D 2O samples were left to equilibrate for at least 10 min before the data were collected. In order to avoid having to obtain hundreds of spectra, we prepared seven stock solutions containing approximately equal amounts of several of our entries, chosen in such a way as to prevent intermolecular interactions and possible ambiguities in assignment. Solution 1: acetone, tert-butyl methyl ether, dimethylformamide, ethanol, toluene. Solution 2: benzene, dimethyl sulfoxide, ethyl acetate, methanol. Solution 3: acetic acid, chloroform, diethyl ether, 2-propanol, tetrahydrofuran. Solution 4: acetonitrile, dichloromethane, dioxane, n-hexane, HMPA. Solution 5: 1,2-dichloroethane, ethyl methyl ketone, n-pentane, pyridine. Solution 6: tert-butyl alcohol, BHT, cyclohexane, 1,2-dimethoxyethane, nitromethane, silicone grease, triethylamine. Solution 7: diglyme, dimethylacetamide, ethylene glycol, grease (engine oil). For D 2O. Solution 1: acetone, tert-butyl methyl ether, dimethylformamide, ethanol, 2-propanol. Solution 2: dimethyl sulfoxide, ethyl acetate, ethylene glycol, methanol. Solution 3: acetonitrile, diglyme, dioxane, HMPA, pyridine. Solution 4: 1,2-dimethoxyethane, dimethylacetamide, ethyl methyl ketone, triethylamine. Solution 5: acetic acid, tertbutyl alcohol, diethyl ether, tetrahydrofuran. In D 2O and CD 3OD nitromethane was run separately, as the protons exchanged with deuterium in presence of triethylamine. (1) For recommendations on the publication of NMR data, see: IUPAC Commission on Molecular Structure and Spectroscopy. Pure Appl. Chem. 1972, 29, 627; 1976, 45, 217. S (97) CCC: $14.00 Figure 1. Chemical shift of HDO as a function of temperature. Results Proton Spectra (Table 1). A sample of 0.6 ml of the solvent, containing 1 µl of TMS, 1 was first run on its own. From this spectrum we determined the chemical shifts of the solvent residual peak 2 and the water peak. It should be noted that the latter is quite temperaturedependent (vide infra). Also, any potential hydrogenbond acceptor will tend to shift the water signal downfield; this is particularly true for nonpolar solvents. In contrast, in e.g. DMSO the water is already strongly hydrogen-bonded to the solvent, and solutes have only a negligible effect on its chemical shift. This is also true for D 2 O; the chemical shift of the residual HDO is very temperature-dependent (vide infra) but, maybe counterintuitively, remarkably solute (and ph) independent. We then added 3 µl of one of our stock solutions to the NMR tube. The chemical shifts were read and are presented in Table 1. Except where indicated, the coupling constants, and therefore the peak shapes, are essentially solvent-independent and are presented only once. For D 2 O as a solvent, the accepted reference peak (δ ) 0) is the methyl signal of the sodium salt of 3-(trimethylsilyl)propanesulfonic acid; one crystal of this was added to each NMR tube. This material has several disadvantages, however: it is not volatile, so it cannot be readily eliminated if the sample has to be recovered. In addition, unless one purchases it in the relatively expensive deuterated form, it adds three more signals to the spectrum (methylenes 1, 2, and 3 appear at 2.91, 1.76, and 0.63 ppm, respectively). We suggest that the residual HDO peak be used as a secondary reference; we find that if the effects of temperature are taken into account (vide infra), this is very reproducible. For D 2 O, we used a different set of stock solutions, since many of the less polar substrates are not significantly watersoluble (see Table 1). We also ran sodium acetate and sodium formate (chemical shifts: 1.90 and 8.44 ppm, respectively). Carbon Spectra (Table 2). To each tube, 50 µl of the stock solution and 3 µl oftms 1 were added. The solvent chemical shifts 3 were obtained from the spectra containing the solutes, and the ranges of chemical shifts (2) I.e., the signal of the proton for the isotopomer with one less deuterium than the perdeuterated material, e.g., CHCl 3 in CDCl 3 or C 6D 5H in C 6D 6. Except for CHCl 3, the splitting due to J HD is typically observed (to a good approximation, it is 1/6.5 of the value of the corresponding J HH). For CHD 2 groups (deuterated acetone, DMSO, acetonitrile), this signal is a 1:2:3:2:1 quintet with a splitting of ca. 2 Hz. (3) In contrast to what was said in note 2, in the 13 C spectra the solvent signal is due to the perdeuterated isotopomer, and the onebond couplings to deuterium are always observable (ca Hz) American Chemical Society

4 Notes J. Org. Chem., Vol. 62, No. 21, For D 2 O solutions there is no accepted reference for carbon chemical shifts. We suggest the addition of a drop of methanol, and the position of its signal to be defined as ppm; on this basis, the entries in Table 2 were recorded. The chemical shifts thus obtained are, on the whole, very similar to those for the other solvents. Alternatively, we suggest the use of dioxane when the methanol peak is expected to fall in a crowded area of the spectrum. We also report the chemical shifts of sodium formate ( ppm), sodium acetate ( and ppm), sodium carbonate ( ppm), sodium bicarbonate ( ppm), and sodium 3-(trimethylsilyl)- propanesulfonate [54.90, 19.66, (methylenes 1, 2, and 3, respectively), and ppm (methyls)], in D 2 O. Temperature Dependence of HDO Chemical Shifts. We recorded the 1 H spectrum of a sample of D 2 O, containing a crystal of sodium 3-(trimethylsilyl)propanesulfonate as reference, as a function of temperature. The data are shown in Figure 1. The solid line connecting the experimental points corresponds to the equation which reproduces the measured values to better than 1 ppb. For the 0-50 o C range, the simpler gives values correct to 10 ppb. For both equations, T is the temperature in C. Acknowledgment. Generous support for this work by the Minerva Foundation and the Otto Mayerhoff Center for the Study of Drug-Receptor Interactions at Bar-Ilan University is gratefully acknowledged. JO971176V δ ) T + ( )T 2 (1) δ ) T (2)

Examination of Proton NMR Spectra What to Look For 1) Number of Signals --- indicates how many "different kinds" of protons are present. 2) Positions of the Signals --- indicates something about magnetic

i Acid-base Chemistry of Aquatic Systems An introduction to the chemistry of acid-base equilibria with emphasis on the carbon dioxide system in natural waters eith A. Hunter Professor in Chemistry Department

The Other CO 2 -Problem Eight Experiments for Students and Teachers introduction Ocean Acidification - The Other CO 2 Problem The oceans cover more than two-thirds of the planet earth. This vast body of

Single-Electron Devices and Their Applications KONSTANTIN K. LIKHAREV, MEMBER, IEEE Invited Paper The goal of this paper is to review in brief the basic physics of single-electron devices, as well as their

New Deal For Young People: Evaluation Of Unemployment Flows David Wilkinson ii PSI Research Discussion Paper 15 New Deal For Young People: Evaluation Of Unemployment Flows David Wilkinson iii Policy Studies

National Renewable Energy Laboratory Innovation for Our Energy Future NREL/TP-540-43672 Revised January 2009 Biodiesel Handling and Use Guide Fourth Edition Notice This report was prepared as an account

All Biochars are Not Created Equal, and How to Tell Them Apart Version 2 (October 2009), which supercedes the digital reprint issued at the North American Biochar Conference, Boulder, CO August 2009 Hugh

Journal of Memory and Language 41, 416 46 (1999) Article ID jmla.1999.650, available online at http://www.idealibrary.com on How to Deal with The Language-as-Fixed-Effect Fallacy : Common Misconceptions

Author Guidelines What to send and when At Submission Please consult individual journal guidelines for details of any additional required files. In general, initial manuscript submissions to Royal Society

Chapter The fluorescence 25 transient as a tool to characterize and screen photosynthetic samples 445 The fluorescence transient as a tool to characterize and screen photosynthetic samples R.J. Strasser,